In the art of vibratory screening apparatus, there are advantages in the use of polymeric screening members. Certain polymers, such as polyurethanes, may be used in lieu of fabricated metal screens in cases where the superior resistance to abrasive wear possessed of polyurethane is an advantage. Polyurethane screening members may be moulded in a wide variety of forms with ease, as opposed to the limitations in fabricating metal screens. Most polyurethane screening members incorporate edge and intermediate reinforcing of steel or other reinforcing to rigidly support the screen surface thereon.
Screening panels may be monolithic, where the screening surface and panel body are moulded monolithically over a frame. Alternatively screens may include a frame over which a poly body is formed, the poly body having upper clipping arrangements provided on the top of the over-moulded frame elements to enable the screening element or “skin” to be removably attached. In such cases, skins may be replaced when worn without the need to replace the panel body. Whilst the skins may be more readily replaceable, due to their design in use they flex and allow the openings through the screen to become distorted which allows larger particles than intended to pass through the screen, which results in the screen failing to grade to a sharp cut-off of particle size.
Australian Patent 559443 (FIORIS) discloses a variety of screen member constructions comprising screen member modules, most having steel reinforced edge portions. The modules are adapted to be assembled in multiples to a support structure, adjacent modules abutting with complementary halves of a bead arrangement adapted to engage a profile support bar which retains the modules in relative location. In one embodiment of FIORIS, there is provided an unreinforced screen panel or skin adapted to snap into a stiffening grid of support bars assembled longitudinally and transversely to a conventional screen deck.
The upper edges of the longitudinal and transverse members have re-entrant section grooves into which corresponding beads of adjacent unreinforced screen panels are inserted and retained thereby. This construction has the advantage that the screen panels are readily replaceable, the worn panels are replaceable, and the screen assembly in use presents an uninterrupted, flat screen surface. The disadvantages are that the screen panels are necessarily small since the span of the flexible polyurethane material comprising the screen panel must be controlled to prevent undue distortion by flexing in use, at reasonable web thicknesses. The smallness of the panels with their peripheral mounting portions reduces the effective screen area. The supporting grid of longitudinal and transverse members is a complicated arrangement of interlocking steel cored polyurethane pieces. The arrangement described in FIORIS is particularly difficult and costly to manufacture.
There are other examples of screening members formed of polyurethanes without reinforcing which are cheaper to manufacture and more easily replaceable than reinforced screens. In Australian Patent Specification AU-A-19011/97 (LETELA) there is provided a modular arrangement whereby a rigid, supporting reinforced polyurethane frame member is adapted to support an unreinforced polyurethane screen panel of large size by virtue of having intermediate support portions. The screen panels are moulded having a peripheral bead adapted to engage with peripheral grooves in the side edges of the frame members to secure the panels thereon. The end portions of the panels and frames overlying the standard-pitch (24″ or approximately) screen deck support bars are adapted to be engaged by and secured to the deck support bars by a lower, bolted-down portion adapted to receive the frame and panel ends, and a locking piece overlaying the panel end edges and engaging the bolted down portion.
This arrangement goes some way to overcoming the disadvantages of the FIORIS apparatus. However, the screening panel is only retained against downward flexing on the intermediate support bar. In practice the screening panels allow the screening web to impact on the intermediate support, which in turn suffers from impact or fatigue tearing of the screening panels that gives the appearance of a cut failure. Polyurethanes are resistant to abrasion wear but are susceptible to cut damage. The screening panels reach sufficient amplitude in upward vibration to cause some loss of grade control in screening. The securing means intrudes onto the plane of the screening surface thus tending to interrupt free flow of particles across the screening surface.
International Patent Publication WO 00/53343 discloses a screening module for a vibratory screen deck including a screen support member that is releasably securable to the screen deck and having a peripheral frame and an intermediate strut, and a polymeric screen member engaged by snap-in connection with each of the peripheral frame portion and the intermediate strut the intermediate strut being located so that flex of the polymeric screen member is controlled.
This construction overcomes the flogging failure mode of the FIORIS and LETELA apparatus. The skins are provided in 1′ square half-panel units, and are able to be inserted with the screening slots oriented either along the 2′×1′ panel frame long axis or transverse to it. However, the skins have to be installed on the panel frames after the frames are installed to a screen machine having a bolt-up panel support frame. Also the system mandatorily eliminates skin flex; the operator does not have the option of allowing skin flex such as where a sticky clay type material is being processed.